Magnetic core assemblies

ABSTRACT

A magnetic core assembly and a method for its manufacture for use in a magnetometer. The core assembly is constructed from a two-piece bobbin. A method is provided for leaving spaces between the ferromagnetic winding and the bobbin to compensate for differences in the thermal coefficient of expansion of the bobbin and the ferromagnetic winding.

Elsie fates afem Weissrnan mlMareli 20, 1973 [54] MAGNET: CORE ASSEMBLllES [75] Inventor: Harold M. Weissman, Lexington,

Mass.

[73] Assignee: Honeywell, lnc., Minneapolis, Minn. 22 Filed: .luly 21, 1967 [21] Appl. No.: 655,124

[52] 11.8. C1. ..242/1l8, 335/297, 336/213 [51] llnt. Cl. B6511 75/12 [58] Field! of Search ..29/605, 606; 336/198, 213,

[56] References Cited UNITED STATES PATENTS 2,949,591 8/1960 Craige ..336/212 X 2,988,674 6/1961 Pizza ..336/2l3 X 3,060,353 10/1962 Shansky et a1.... ..33 6/213 X 12/1962 Jones .1336/213 Primary Examiner-George F. Mautz Attorney-Roger W. Jensen, Charles J. Ungemach and Bruce C. Lutz [57] ABSTRACT A magnetic core assembly and a method for its manufacture for use in a magnetometer. The core assembly is constructed from a two-piece bobbin. A method is provided for leaving spaces between the ferromagnetic winding and the bobbin to compensate for differences in the thermal coefficient of expansion of the bobbin and the ferromagnetic winding.

4 Claims, 3 Drawing Figures PATENTEUIMRZOIBB 31721 ,400

SHEET 10F 2 INVENTOR. HAROLD M. WEISSMAN FIG. I f 7 ATTORNEY PATENTEUHARZOISH SHEET REF 2 JL l FIG. 3

INVENTOR.

HAROLD M. WEISSMAN BY M ATTORNEY MAGNETIC CORE ASSEMBLIES BACKGROUND This invention pertains to the manufacture of ferromagnetic core assemblies. Ferromagnetic cores are well known in the art and have numerous applications. The use of ferromagnetic material wound on a bobbin for a magnetometer sensor core has been known in the prior art. The sensitivity of a magnetometer makes it very susceptible to changes in the magnetic characteristics of the sensor. After exposure of magnetometer sensors to temperature extremes, permanent changes in the magnetic characteristics of the sensor often result. These changes in sensor characteristics are introduced by stresses applied to the ferromagnetic core when the bobbin tends to contract less with a decrease in temperature than does the tightly wound ferromagnetic ribbon. The resulting stress in the ferromagnetic material causes permanent and not entirely predictable shifts in the characteristics of the ferromagnetic material.

The present invention provides a solution to the problem introduced by the difference between the thermal expansion of coefficients of a ferromagnetic ribbon and the bobbin about which it is wound.

DESCRIPTION The core assembly is constructed so that at extreme temperature the differing thermal expansion coefficients of the ferromagnetic windings and the bobbin material will not apply stress to the ferromagnetic ribbon. If the thermal coefficients of expansion of the bobbin and the ferromagnetic ribbon are exactly equal, no mechanical stresses will be introduced at temperature extremes. Since it is improbable that a practical magnetically inactive material can be found with a coefficient of expansion exactly equal to that of a ferromagnetic ribbon, addition steps must be taken to compensate for the differences in thermal coefficients of the two materials.

The bobbin is produced in two telescoping sections and one or several thin strips of Teflon or like material are inserted before winding the ferromagnetic ribbon around the internal portion of the bobbin. After the winding has been completed, the Teflon may easily be withdrawn before the two halves of the bobbin are assembled. Removal of the spacer strips at completion of the winding leaves a small space between the bobbin and the winding which allows for the unequal expansion and contraction of the ferromagnetic winding and the bobbin when temperature extremes are encountered.

It is an object of this invention to provide a core for a magnetometer sensor which is not sensitive to temperature induced offsets.

It is a further object of this invention to provide a simple method for producing a magnetic core assembly with controlled space between a ferromagnetic ribbon and the supporting bobbin structure.

Further objects and advantages will become apparent from a reading of the specification and claims in conjunction with the drawings wherein:

FIG. 1 isa top view of the assembled core assembly.

FIG. 2 is a cross-sectional view of the assembled core assembly taken along section lines 2-2 of FIG. 1.

FIG. 3 is a pictorial view of the internal bobbin portion illustrating the use of a Teflon lamina during the winding of a ferromagnetic material to provide a space between the winding and bobbin after removal of the Teflon.

In, FIG. 1 an internal bobbin portion 11 and an exter nal bobbin portion 22 are shown in their assembled positions. In the preferred embodiment the bobbin portions are formed from Supra-Mica 555. Because it has a temperature coefficient similar to that of the ferromagnetic material. The internal portion 11 has an outline when viewed in elevation which is defined by two inwardly facing half circles 13 which are connected by parallel lines 14 whose length is much greater than the radius of half circle 13 and which are tangent thereto. Completely and symmetrically enclosed within the area defining the top surface of the internal bobbin portion 11 is an aperture 12 of which the perimeter seen in elevation is described by a first set of parallel lines 15 which are parallel to the parallel lines 14, and a second set of parallel lines 16 perpendicular to the first set of parallel lines 15 and passing through the center point of each of the half circles 13. The parallel lines 15 are connected to the second set of parallel lines 16 by arcuate segments 17 which have a radius considerably smaller than the radius defined by the half circles 13.

The external bobbin portion 22 as shown in FIG. 1 has an outer perimeter defined roughly by a rectangle with corners defined by arcuate segments 18. The outer perimeter 14 of the internal bobbin portion 11 is enclosed within the inner perimeter of the external bobbin portion 22 leaving a very small space between the internal bobbin portion 1 1 and the external bobbin portion 22. The space is exaggerated for the purposes of clarity of the drawing.

In FIG. 2 the internal bobbin portion 11 is shown inserted into the external bobbin portion 22. External bobbin portion 22 is formed with a curved outer surface 23 to facilitate the winding of coils about the core assembly to form a magnetometer sensor. An inner surface 25 of the external bobbin portion 22 is distinguished by an inwardly oriented annular step 24 which is located near a top surface 19 of the bobbin assembly. An inner wall 25'of the external bobbin portion 22 has approximately a one degree slope opening toward a bottom surface 29 of the core assembly.

The internal bobbin portion 11 is constructed with a first outwardly oriented annular step 20 constructed so as to interface with the inwardly oriented annular step 24 of the external bobbin portion 22. A second outwardly oriented annular step 21 is constructed such that when the'intemal bobbin portion 11 and the external bobbin portion 22 are interfaced, a residuum of space is left between the inner surface 25 of the external bobbin portion 22 and an outer surface 26 of the internal bobbin portion 11. This space is bounded by the first outwardly oriented annular step 20 and the second outwardly oriented annular step 21 and walls 25 and 26. The aperture 12 of the internal bobbin portion 11 appears in cross section as a combination of three isosceles trapezoids. The first isosceles trapezoid 27 is located with its longer parallel side parallel with the top surface 19 of the bobbin assembly and its non-parallel sides intersect the longer parallel side at approximately a 45 angle. The second isosceles trapezoid 28 has its longer parallel side parallel to the bottom surface 29 of the bobbin assembly and its non-parallel sides intersect the longer parallel side at approximately a 45 angle. The third isosceles trapezoid 30 shares its parallel sides with the shorter parallel sides of the first isosceles trapezoid 27 and the second isosceles trapezoid 28. The non-parallel sides of the third isosceles trapezoid 20 are constructed to form a gentle linear taper outward from the bottom of the core assembly with a taper angle of about 1 to allow for mold release.

In FIG. 3 a lamina 31 is shown in its preferred position against the internal bobbin portion 11 with one end resting on the second outwardly oriented annular step 21 and with the surface of the lamina engaging the outer surface 26 of the internal bobbin portion 11. The preferred embodiment uses a Teflon lamina because of its desirable low friction characteristics. Ferromagnetic material 32 is wrapped several times around the internal bobbin portion 11 in the area between the first outwardly oriented annular step 20 and the second outwardly oriented annular step 21 and covering the surface 26 of internal bobbin portion 11 as well as the lamina 31. After the ferromagnetic material 32 has been wound about the internal bobbin portion 11, the Teflon lamina 31 is removed to leave a small residuum of space to provide for the difference between the coefficient of expansion of the internal bobbin portion 11 and the ferromagnetic material 32.

The internal bobbin portion 11 with the ferromagnetic material wound loosely thereon is then inserted into the external bobbin portion 22. The assembled bobbin then has an appearance as in FIGS. 1 and 2 except that the ferromagnetic material 31 loosely occupics the space between surfaces 25 and 26 in FIG. 2. A toroid of electrically conducting material is wound about the assembled bobbin to enclose the flux path formed by the ferromagnetic material 31.

Although the preferred embodiment of this core assembly has been shown to be an elongated annulus, the words core assembly as used here apply not only to toroidally shaped annular core assemblies but to a core assembly of any shape which provides a closed ferromagnetic path with an aperture through the area enclosed by the path so that windings can encircle the ferromagnetic flux path. Although the preferred embodiment of the bobbin is constructed from Supra-Mica 555 because of its desirable temperature coefficient it is obvious that any ceramic material which is magnetically inactive may be substituted. The lamina is formed from Teflon so that it may easily withdraw but it may be replaced with lamina of other materials.

Other alterations and variations will be obvious to those skilled in the art. I do not wish to be limited to the specification of the preferred embodiment as shown in the figures but only the following claims.

I claim:

1. A bobbin comprising an elongated rigid hollow annulus of magnetically inactive material, including internal and external interfitting portions;

said external portion having an aperture with an inwardly oriented annular step; and

said internal portion having a first outwardly oriented annular step which lies within the aperture of said external portion when the portions are assembled, the inwardly oriented annular step and the first outwardly oriented annular step cooperat- 3. The apparatus as defined in claim 2 wherein said internal portion has a second outwardly oriented annular step, said second step contacting the inwardly oriented annular step of said external portion when the portions are assembled.

4. The apparatus as defined in claim 3 wherein said outer surface lies between said first and second annular steps. a

tnrrrn STATES rarrmr orrrcr @ETEFKQATIE Qt @QREQHQN Patent No. 3 721.,400 Dated March 20. 1973 'Inventor(s) Harold M. Weissman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Drawings, FIG. 3 please delete reference numeral 27 and insert in its place reference numeral 21; and

Same Figure, please add a lead-line running from reference numeral 26 to the surface of inner bobbin portion ll between annular steps 20 and 210 Signed and sealed this 25th day of December 1973.

SEAL) Attest:

EDWARD M.FLETQHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM PO-1050 (10-69) USCOMM-DC 60376-1 69 I a u,s. GOVERNMENT Pmmmc omcz; Ins o-qu-au f UNWE STATES PATENT erwen QER'HMQATE @E @QRREQlW-N Patentlkn 3,721,400 Dated March 20 1973 Inventor(s) Harold M. Weissman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Drawings, FIG 3, please delete reference numeral 27 and insert in its place reference numeral 21; and

Signed and sealed this 25th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM PC4050 (10-69) 

1. A bobbin comprising an elongated rigid hollow annulus of magnetically inactive material, including internal and external interfitting portions; said external portion having an aperture with an inwardly oriented annular step; and said internal portion having a first outwardly oriented annular step which lies within the aperture of said external portion when the portions are assembled, the inwardly oriented annular step and the first outwardly oriented annular step cooperating in assembly to comprise an annular chamber totally enclosed within the boundaries of said portions.
 2. The apparatus of claim 1, wherein said internal portion has an outer surface which, when viewed in elevation, has an outline defined by two inwardly facing half circles connected by parallel lines.
 3. The apparatus as defined in claim 2 wherein said internal portion has a second outwardly oriented annular step, said second step contacting the inwardly oriented annular step of said external portion when the portions are assembled.
 4. The apparatus as defined in claim 3 wherein said outer surface lies between said first and second annular steps. 